41 Egipto.pdf

Alexandria Engineering Journal, Vol. 43 (2004), No. 1, 41-54 41 Faculty of Engineering, Alexandria University, Egypt.

Strip footing behavior on pile and sheet pile-stabilized sand slope

Mostafa A. El Sawwaf Structural Eng. Dept., Faculty of Eng., Tanta University, Tanta, Egypt

This paper presents the results of laboratory model tests on the behavior of a strip footing supported on a row of piles and sheet pile-stabilized earth slope. A comparison between the bearing capacity improvements in the two cases was made to study the most efficient case. The parameters varied in the study include pile diameter, pile length, pile spacing and location of pile row, height of sheet pile, location of sheet pile and location of the footing relative to the slope crest. Initially the bearing capacity of non-stabilized cases were determined and then compared with those of stabilized slopes. The results were then analyzed to study the effect of each parameter. The results indicate that stabilizing earth slope using a row of piles or sheet pile has a significant effect in improving the bearing capacity of the strip footing. This improvement in bearing capacity increases when pile spacing decreases and pile length increases with further improvement with increasing pile diameter. However, the overall improvement when using sheet pile to stabilize earth slope is much better than that when using a row of piles. Finally, a series of finite element analysis was performed on a prototype slope and a comparison between the laboratory model tests and the finite element analysis was presented. ! "! !#$ % ! & % ! '! & ' (!)! & *+ !) (,%,! - %

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Keywords: Pile, Sheet pile, Stabilized sand slope, Strip footing, Finite element analysis

1. Introduction

There are many situations where footings

are constructed on sloping surfaces or adja-cent to a slope crest such as footings for bridge abutments on sloping embankments. When a footing is located on a sloping ground, the bearing capacity of the footing may be sig-nificantly reduced, depending on the location of the footing with respect to the slope. There-fore it may not be possible to use shallow fou-ndation and using uneconomic foundation types (piles or caissons) becomes the only suitable solution of the problem. Therefore, over years, the subject of stabilizing earth slope has become one of the most interesting areas for scientific research and attracted a great deal of attention. Slope stability can be

increased in different ways such as: modifying the slope surface geometry, using soil reinfor-cement, or installing continuous or discrete retaining structures such as walls or piles. There have been numerous studies on the use of slope reinforcement to improve load bearing capacity of a footing on the slope (Selvadurai et al. [1], Sawicki et al. [2], Mandal et al. [3], Huang et al. [4], Zornberg et al. [5] and Yoo C. [6]). These investigations have demonstrated that not only the slope stability can be increased but also both the ultimate bearing capacity and the settlement characteristics of the foundation can be significantly improved by the inclusion of reinforcements (layers of geogrid, strips or geotextile) in the earth slope.

On the other way, the use of stabilizing piles to support active earth slope has been

M. A. El Sawwaf / Strip footing behavior

42 Alexandria Engineering Journal, Vol. 43, No. 1, January 2004

considered to be one of the important slope reinforcement techniques in the last few dec-ades. These piles, which can be driven at the crest or within the slope itself act as resisting members and are usually subjected to lateral forces by the horizontal movements of the sur-rounding soil. The lateral force acting on each pile may be obtained in an approximate man-ner multiplying the resisting force per unit width of the pile row by the center-to-center spacing between the piles. However, to evalu-ate the force acting on the piles more accu-rately, arching between adjacent piles should be considered. Several studies reported the successful use of piles in many situations in order to improve slope stability (De Beer and Wallays [7], Viggiani [8], Poulos [9], Lee et al. [10], Hong and Han [11], Chen et al. [12], Has-siotis et al. [13], Ausilio, et al. [14] and Hull and Poulos [18]).

Poulos [9] described an approach for the design of piles to reinforce slopes, in which both the total shear force needed to increase the safety factor and the maximum shear force that each pile can provide were evaluated and therefore the type, number of piles, and the most suitable locations of these piles within the slope can be selected. Hassiotis et al. [13] proposed a method for the design of slopes reinforced with a single row of piles based on the theory of plasticity to find the lateral forces acting on the pile section above the critical surface. Chen et al. [12] presented a theoreti-cal procedure for analyzing the lateral re-sponse of vertical piles subjected to lateral soil movements via a simplified boundary-element analysis. Ausilio et al. [14] reported that when a row of piles is inserted in a slope, the addi-tional resistance provided by these piles changes both the slope safety factor and pot-ential failure mechanism with respect to the case without piles. Several studies have been conducted in or-der to find out the best location of the stabiliz-ing piles row within a slope. This was achieved by determining the position of the piles row that gives the maximum resistance force and factor of safety. However, the recommended results are rather different and contrasting. Ito

and Matsui [15] in their analytical studies demonstrated that a row of piles installed closer to the top of the slope gives the best

factor of safety but in (1979) Ito et al. [16] showed that the best location of piles row, which has the maximum effect on slope stability, is the upper-middle part of the slope. Hassiotis et al. [13] concluded that the piles should be located close to the top of the slope to achieve the maximum safety factor. Lee et al. [10] analyzed cohesive soil slope and found that when the piles are installed into a homo-geneous soil the most effective pile positions are the toe and crest of the slope and in contrast to other researchers, stated that the piles have little effect on slope stability when they are located close to the middle of the slope. Cai and Ugai [17], using the finite element method, have pointed out that the maximum safety factor for the slope can be achieved when the piles are located in the middle of the slope.

It should be mentioned that most of the previous studies concerning the pile-stabilized slope have aimed at the stability analysis of the slope itself. However, the improvement in the load bearing capacity of a footing sup-ported on sand bed adjacent to stabilized slope hasnt yet been investigated. Therefore the aim of this study is to gain more under-standing about the mechanical behaviour and the failure mechanism of a strip footing sup-ported on sand bed adjacent to pile or sheet pile stabilized earth slope. The main objective was to determine and establish the relation-ship between the variable parameters of a row of piles or sheet pile and the bearing capacity of the footing. Also, to find out the best loca-tion of the piles row or sheet pile that gives the best improvement in the footing bearing cap-acity. So, series of experimental model tests were carried out and the obtained results are presented and discussed. Also, a series of finite element analysis was performed on a prototype slope to ascertain the validity of the findings from the experimental model tests and understand the deformation patterns of soil particles underneath the footing.

2. Laboratory model tests 2.1. Model box and footing

The experimental work aimed to study the effects of stabilizing an earth slope on the


Alexandria Engineering Journal, Vol. 43, No. 1, January 2004 43

225.0

10.0 10.0

60 960 60

805

800

50

.0 c

m

90.0 cm

1

2

3

4

5

6

89

125.0

1- Manual wench

2- Wench wire

3- Raining box

4- Hydraulic jack

5- Proving ring

6- Model footing

7- Soil

8- Test tank

9- Loading frame

10- Dial gauges

7

Dimensions are in centimeters

10

bearing capacity of a strip footing adjacent to the slope crest. A plane strain footing was used in the study while a row of piles and sheet pile wall were used to stabilize the earth slope. The main elements of the used apparatus are a tank, a horizontal steel beam over the tank, and a sand-raining box. The test box, having inside dimensions of 1.00 m * 0.50 m in plan and 0.5 m in depth is made from steel with the front wall made of 20 mm thickness glass and is supported directly on two steel columns as shown in fig. 1. The glass side allows the sample to be seen during preparation and sand particle deformations to be observed during testing. The tank box was built sufficiently rigid to maintain plane strain conditions by minimizing the out of plane displacement. To ensure the rigidity of the tank, the back wall of the tank was braced on the outer surface with two steel beams fitted horizontally at equal spacing. The inside walls of the tank are polished smooth to reduce friction with the sand as much as possible by attaching fiber glass onto the inside walls.

The loading system consists of a hand-operated hydraulic jack and pre-calibrated load ring. Since the sand raining technique is used to deposit the sand inside the tank, the beam was designed to swing about one end. Therefore, the beam can be swung out during sand deposition and returned back, when sand set up completed, to the original loading position. The sand raining box is made from wood and is 0.85 m * 0.48 m in plane and 0.10 m depth. The sand particles rain from the box through a square grid of holes (4 mm diameter and 20 mm spacing) in the base plate. The height of sand raining, measured from the bottom of the box to sand surface in the tank, could be changed up or down by using a manual winch.

A strip model footing made of steel with a hole at its top center to accommodate bearing ball was used. The footing is 498 mm in length, 80 mm in width and 20 mm in thickness. The footing was positioned on the sand bed with the length of the footing running the full width of the tank. The length of the footing was made almost equal to the width of the tank in order to prevail plane strain conditions within the test set-up. The two ends of the footing plate were polished

smooth to minimize the end friction effects. A rough base condition was achieved by fixing a thin layer of sand onto the base of the model footing with epoxy glue. The load is transferred to the footing through a bearing ball as shown in fig. 1. Such an arrangement produced a hinge, which allowed the footing to rotate freely as it approached failure and eliminated any potential moment transfer from the loading fixture.

2.2. Test material

The sand used in this research is medium

to coarse sand, washed, dried and sorted by particle size. It is composed of rounded to sub-rounded particles. The specific gravity of the soil particles was determined by the gas jar method. Three tests were carried out and the average value was obtained. The maximum and the minimum dry densities of the sand were measured and the corresponding values of the minimum and the maximum void ratios

Fig. 1. Schematic view of the experimental apparatus.



Table 1 The general physical characteristics of sand

Coefficient of uniformity 4.071

Effective diameter (m) 0.152

Coefficient of curvature (Cc) 0.771 Specific gravity, (Gs) 2.654 Maximum unit weight (kN/m3) 19.95 Minimum unit weight (kN/m3) 16.34 Maximum void ratio 0.593 Minimum void ratio 0.305

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10 100

Grain size, mm

perc

en

t fi

ner

Fig. 2. Grain size distribution of the used sand.

were calculated. The particle size distribution was determined using the dry sieving method and the results are shown in fig. 2. Table 1 summarizes the general physical characteris-tics of the used sand.

Sand beds were placed in -50 mm in height- layers by raining technique in which sand is allowed to rain through air at controlled discharge rate and height of fall to give uniform densities. The relative density achieved during the tests was monitored by collecting samples in small cans of known volume placed at different locations in the test tank. The raining technique adopted in this study provided a uniform relative density of approximately 75.8 % with a unit weight of 18.94 kN/m3. A series of direct shear tests was performed to evaluate the shear strength properties of the model ground using specimens prepared by dry tamping. The estimated internal friction angle at the same relative density used in the model tests was 42.

Model piles made of steel with different lengths and diameters were used in the study.

The piles were 6.0, 8.0, and 12.0 mm in diameters and 80, 100, 160, 240 mm in length. Sheet piles were 3.0 mm in thickness and made of wood with different heights. All the tests were performed with piles or sheet pile installed vertically after preparing sand samples. The difference in the relative densi-ties of the samples which occurs during inst-alling pile or sheet pile wall due to the diff-erence in the pile lengths or sheet pile heights was considered to be small and neglected. 2.3. The experimental setup and test program

The model soil was constructed by raining

technique with the bed level and slope observed through the front glass wall. Then the sand slope was set up to form a slope of 3(H): 2 (V). In raining the last layer, the sand surface was approximately horizontal and the sand slope was about the required inclination. Great care was given to level the top surface of the sand and the slope face using special rulers so that the relative density of the top layer was not affected. After setting up the model slope, a row of piles or the sheet pile was driven vertically at the design place and spacing. The footing was placed on position and the load was applied incrementally by the hydraulic jack until reaching failure. Each load increment was maintained constant till the footing settlement had stabilized. This settlement was measured using two 0.001 mm accuracy dial gauges, placed on opposite sides across the center of the footing.

A total of 80 tests in two different test programs were carried out. Initially, the response of the model footing supported on the non-stabilized slopes was determined (2 tests, each one was repeated 3 times). Then, 15 series of tests (60 tests) were performed to study the effect of the different parameters of a row of piles on the footing behaviour as shown in fig. 3.a. Each series was carried out to study the effect of one parameter while the other variables were kept constant. The varied conditions include the pile diameter D, pile length L, pile spacing x and the horizontal distance between pile row and the slope crest d as illustrated in table 2. Finally, in the sheet pile test program, 4 series of tests (18 tests) were performed to study the footing response



3

2

Bbd

Footing

PileL

x

x

x

H = 4.5 B

0.5 B0.5 B

3

2

Bbd

Footing

Sheet Pile

h

0.5 B0.5 B

H = 4.5 B

(a) (b)

Fig. 3. Geometric parameters of pile and sheet pile-stabilized sand slope model.

Table 2 Model tests program

Series Constant parameters Variable parameters Notes

A Tests on non-stabilized sand slope. b = 0.0 3 times

B Tests on non-stabilized sand slope. b = B 3 times

1 D/B = 0.075 & d/B = 0.0 & L/B = 1.00 x/B = 0.5, 1.00, 1.25, 2.5 piles row












13 D/B = 0.075 & x/B = 0.5 & L/B = 2.00 d/B = 0.0, 1.0, 1.5, 2.0, 2.5 piles row



I b/B = 0.0 & d/B = 0.0 h/B = 0.5, 1.0, 1.5, 2.0, 3.0 sheet pile

II b/B = 1.0 & d/B = 0.0 h/B = 0.5, 1.0, 1.5, 2.0, 3.0 sheet pile

III b/B = 0.0 & h/B = 2.0 d/B = 0.0, 1.0, 1.5, 2.0, 2.5 sheet pile

IV b/B = 1.0 & h/B = 2.0 d/B = 0.0, 1.0, 1.5, 2.0, 2.5 sheet pile

for the two cases when the footing is placed exactly on the slope crest and when the footing is placed away of the slope crest by a distance b. For each case, two series of tests

were conducted to find out the best location of the sheet pile that gives the maximum bearing capacity improvement and the effects of sheet

pile height h as shown in fig. 3-b. Table 2

summaries all the tests programs with both the constant and varied parameters illus-trated. Several tests were repeated at least twice to verify the repeatability and the consis-tency of the test data.



3. Finite element analysis

A series of numerical analysis on a proto-type footing-slope system was carried out using finite element method. The object of this series is to verify the laboratory model tests results and to understand the particles defor-mations within the soil mass. The analysis was performed using the PLAXIS software package (professional version 7, Bringkgreve and Vermeer [19]). The geometry of the proto-type footing-slope system was assumed to be 10 times the laboratory model (B = 0.8 m, H =

3.60 m, the thickness of sheet pile wall = 0.30 m). The modeled boundary conditions were as-sumed such that the vertical boundaries are free vertically and constrained horizontally while the bottom horizontal boundary is fully fixed. The same inclination of model test slopes, 3 (H): 2 (V), and the material of sheet pile wall (wood) were used in the prototype study. The software allows the automatic gen-eration of six or fifteen node triangle plane strain elements for the soil, and three or five node beam elements for the footing and the sheet pile wall. The analyzed prototype slope geometry, generated mesh, and the boundary conditions are shown in fig. 4.

The non-linear behavior of sand was mod-eled using hardening soil model, which is an elastoplastic hyperbolic stressstrain model, formulated in the framework of friction hard-ening plasticity. A basic feature of the hyper-bolic model is the stress dependency of soil stiffness. The limiting state of stress are de-scribed by means of the secant Youngs

modulus ref50

E , tangent stiffness modulus for

primary compression refoed

E , Poissons ratio ,

power for stress-level dependency of stiffness m, effective cohesion c, angle of internal

friction , angle of dilatancy , failure ratio Rf and interface reduction factor Rint. The sheet

pile wall and the footing were modeled using elastic beam elements based on Mindlins beam theory with significant flexural rigidity and normal stiffness. The interaction between the sheet pile wall and soil is modeled at both sides by means of interface elements, which allow for the specification of a reduced wall friction compared to the friction of the soil. Load control method was used to apply a

prescribed load in increments accompanied by iterative analysis up to failure. As the slope surface is not horizontal, the gravity loading was applied to calculate the initial stress field of the soil in steps with the sheet pile wall in place. An internal angle of friction and secant

Youngs modulus ref50E of 40 and 40000

kN/m2 derived from a series of drained triaxial compression tests were used for the soil material along with the hyperbolic parameters for the sand taken from data base provided by the software manual as shown in table 3.

4. Results and discussion

The bearing capacity improvement of the footing due to slope stabilization is repre-sented using a non-dimensional factor, called Bearing Capacity Improvement factor (BCI). This factor is defined as the ratio of the footing ultimate pressure with the slope stabilized qu stabilized to the footing ultimate pressure in tests without slope stabilizing qu. The footing settlement S is also expressed in non-dimen-sional form in terms of the footing width B as the ratio S/B %. The ultimate bearing capaci-

ties for the footing-soil systems are determined from the loaddisplacement curve as the pro-nounced peaks, after which the footing col-lapses and the load comes down. The meas-ured ultimate bearing capacity for non-stabi-lized cases when the footing was placed at dis-tances d = 0.0 and d = B from the slope crest are 17.30 and 36.69 kPa, respectively. 4.1. Pile-stabilized slopes Fig. 5 presents typical variations of BCI with settlement ratio S/B for a strip footing

supported on sand bed adjacent to both stabi-lized and non-stabilized earth slope (series 1). In this series all the variable were kept con-stant but pile spacing varied. It is clear that the installation of a row of piles much im-proves both the initial stiffness (initial slope of the loadsettlement curves) and the bearing load at the same settlement level for stabilized case compared to the non-stabilized earth slope. Also, the bearing capacity improve-ments at failure are significantly dependent on the pile spacing. However, the curves show that these improvements in bearing capacity



pile diameter = 6 mm

0

1

2

3

4

5

6

7

8

0 0.25 0.5 0.75 1 1.25 1.5 1.75

BCI

S// //B

, %% %%

no piles

x/B = 2.5

x/B = 1.5

x/B = 1.0

x/B = 0.5

The footing

The sheet pile wall

Fig. 4. The prototype slope geometry, generated mesh, and boundary conditions .

Table 3 Hardening soil model parameters used in the finite element analysis

Parameter Value

Primary loading stiffness, (ref50E ), kN/m

2 40000

Cohesion (c), kN/m2 0.00

Friction angle, 40.0

Dilatancy angle, 10

Soil unit weight (), kN/m3 18.90

Power in stiffness law, m 0.70 Failure ratio Rf 0.90 Interface reduction factor (Rint) 0.8

are accompanied with an increase in the settlement ratio. Comparing the curves across the dotted line for the same level of settlement ratio, the figure illustrates that using a row of piles with x/B = 0.5 at the slope crest brings

out an increase in the bearing capacity of 50% more than that of non-stabilized case. The ultimate bearing capacities for different cases of earth slopes stabilized by a row of piles are given in table 4-a and 4-b. These results for each parameter are discussed in the following sections. 4.1.1. The effect of pile diameters

In order to study the effect of pile diameter, a series of tests using a row of piles installed at slope crest with the same normalized pile length L/B=3 with pile diameters D of 6.0, 8.0, and 12.0 mm were

carried out. Fig. 6 illustrates the variations of bearing ca BCI with settlement ratio S/B for a footing placed near to the crest b/B=0. The

figure clearly shows that the footing perform-ance much improves with the increase in pile diameter. It can be seen that an improvement as high as 2.03 % more than that without piles can be obtained when using normalized

pile diameter D/B of 0.15. This can be

illustrated that as the pile diameter increases its resistance to the lateral movement of soils under the footing increases and hence the bearing capacity ratio improves. Also, for the same number of piles in a row, increasing the pile diameter decrease the clear distance be-tween piles. Therefore, not only less soil particles are likely to move through but also the pile resistance for lateral movement by arching effect is getting higher leading to better performance of the pile row as retaining system of the lateral movements.

Fig. 5. Typical variations of BCI with S/B for different pile spacing (Series 1).

4.1.2. The effect of pile length Fig. 7 shows the variation of bearing

capacity improvement factor (BCI) with normalized pile length (L/B) for a footing placed near to the slope crest (b/B=0) and

using a row of piles installed at slope crest. The three curves demonstrate the same pattern of footing response for the different pile diameters. As the pile length increases, the improvement in bearing capacity is becoming higher. Using a row of piles with x/B = 0.5 and D/B =0.15 with normalized pile length of L/B = 3 instead of L/B = 1 increases

the improvement in bearing capacity by 20 %(rom 1.75 to 2.1).



Table 4-a Ultimate bearing capacity (kPa ) from model test results of pile stabilized slope program

X/B Pile

diameter L/B

0.5 1.0 1.25 2.5

1.0 27.25 23.33 22.01 18.08

1.25 28.83 24.11 23.06 18.61

2.0 29.88 25.16 24.64 19.40 6 mm

3.0 33.02 28.31 26.21 20.44

1.0 28.83 24.64 23.06 18.35

1.25 29.88 25.69 24.11 18.87

2.0 31.45 27.25 26.21 19.92 8 mm

3.0 34.60 29.88 27.26 21.49

1.0 30.40 26.52 24.90 19.24

1.25 31.45 28.30 25.69 19.92

2.0 33.02 29.36 26.73 20.97 12 mm

3.0 35.12 31.45 28.83 22.02

Table 4-b Ultimate bearing capacity (kPa ) from model test results of the series of pile row location (Series 13,14 and 15)

d/B Pile

diameter 0.0 1.0 1.5 2.0 2.5

6 mm 29.88 27.78 25.69 22.54 20.44

8 mm 31.45 28.83 26.21 23.59 20.97

12 mm 33.02 29.88 26.73 24.64 22.02

This increase in pile length results in increasing the part of pile embedded in the stable underlying soils leading to more stability for the pile and higher resistance for the lateral movement of soil particles under the footing.

4.1.3. The effect of pile spacing

In order to investigate the effect of pile spacing on the footing performance, a row of piles with four different pile spacing of 40.0, 80.0, 100.0 and 200 mm were used. Fig. 8 shows the variations of BCI with normalized pile spacing x/B for different pile lengths. A

significant increase in the bearing capacity of the model footing supported on stabilized sand

1

1.25

1.5

1.75

2

2.25

0 0.05 0.1 0.15 0.2

(D/B)

BC

I

x/B = 0.50

x/B = 1.00

x/B = 1.25

x/B = 2.50

Fig. 6. Variations of (BCI) with normalized pile diameter (D/B) for different pile spacing (b/B=0, L/B=3.0).

1

1.25

1.5

1.75

2

2.25

0 0.5 1 1.5 2 2.5 3 3.5

(L/B)

BC

I

D/B = 0.15

D/B = 0.10

D/B = 0.075

Fig. 7. Variations of (BCI) with normalized pile length (L/B) for different pile diameters (d/B = 0 and x/B = 0.5).

slope with the decrease of normalized pile spacing x/B can be noticed. Using a row of piles with normalized pile spacing x/B of 0.5 and normalized pile length L/B =3 could bring

out an improvement in bearing capacity as high as 2.12 times more than that without slope stabilizing. However, for the same row of piles with normalized pile spacing x/B of 2.5,

the improvement in bearing capacity is only 1.28. Therefore, decreasing the pile spacing from x/B of 2.5 to x/B of 0.5 increases the

improvement in the bearing capacity by about 65 %.



1

1.25

1.5

1.75

2

2.25

0 0.5 1 1.5 2 2.5 3

(x/B)

BC

I L/B = 3.00

L/B = 2.00

L/B = 1.25

L/B = 1.00

Fig. 8. Variations of (BCI) with normalized pile spacing (x/B) for different pile lengths (d/B = 0 and D/B = 0.15).

This expected increase in the bearing capacity of the footing can be explained for the following two reasons. The first is that when pile spacing is increased, the clear distance between piles increases allowing more soil to move through and larger lateral movement of soil to occur under the footing. The second is that for small pile spacing the component of pile resistance for lateral movement by arch-ing effect is higher leading to a decrease in the lateral movement of the soil and hence incr-easing both the slope stability and the footing bearing capacity. 4.1.4. The effect of the location of pile row

It was shown that there was some confu-sion among researchers about the optimal lo-cation of piles row that gives the best stability of a slope and the maximum factor of safety. Different locations were reported such as the slope crest and the slope toe while others stated that the row placed in the middle part is the best location. However, it was of interest to find out experimentally the best location of pile row from the point of view of the footing bearing capacity rather than the overall stability of the slope. So, series of tests using the same row of piles (x/B =0.5 and L/B = 2)

at five different locations relative to the slope crest (d = 0.0, 80.0, 120.0, 160 and 200.0

mm) were carried out using different pile diameters. Fig. 9 shows the variations of BCI with normalized pile row location d/B for

different pile diameters. It can be seen that as the pile row is placed nearer to the slope crest the response of the footing is getting much better in terms of bearing capacity than any-where else. The same trend is confirmed by the different series carried out using different pile diameters.

This conclusion that the most effective pile location is at the slope crest can be referred to the fact that the passive wedge under the footing is relatively shallow and hence the mo-bilized passive resistance is getting much higher when the pile row are placed at the crest. Any other position far from that location may increase the overall stability of the slope but cant prevent or decrease the lateral defor-mations of soil particles under the footing and near to the slope.

In order to understand the mechanism of footing failure and whether or not it was acc-ompanied with slope failure, additional incre-ments of loads were applied after failure point (the pronounced peak in the load - settlement readings) and both the footing and the slope were observed through the front glass wall. It was noticed that, as the footing approached failure the vertical settlements were accompa-nied with horizontal movements and rotations (toward the slope). It is very important to men-tion that in all the experimental tests, there wasnt any failure observed in the slope itself even after footing settlement S/B ratio of 50%. 4.2. Sheet pile stabilized sand slope

The improvement in footing response due

to slope stabilization by sheet piles with different heights h and locations relative to the slope crest d were investigated. The bearing capacity improvement factor BCI along with the footing settlement ratio S/B were used to

present the test results. Table 5-a and 5-b gives the ultimate bearing loads for the different studied conditions. 4.2.1. The effect of sheet pile height Two series of tests were performed on sand slope stabilized with sheet pile placed at slope



1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

0 0.5 1 1.5 2 2.5 3(d/B)

BC

I

D/B = 0.15

D/B = 0.10

D/B = 0.075

Fig. 9. Variations of BCI with normalized pile row location d/B for different pile diameters (x/B = 0.5 and L/B = 2.0).

Table 5-a Ultimate bearing capacity (kPa ) from model test results of the series of sheet pile height (d/B=0.0)

h/B

b/B 0.5 1.0 1.5 2.0 3.0

0.0 29.88 32.50 34.34 37.48 45.34

1.0 38.27 40.62 42.20 45.34 49.54

Table 5-b Ultimate bearing capacity (kPa ) from model test results of the series of sheet pile location (h/B=2.0)

d/B

b/B 0.0 1.0 1.5 2.0 2.5

0.0 37.48 34.07 30.93 26.73 20.97

1.0 45.34 43.51 42.46 40.89 39.31

crest (d/B = 0.0) to find out the effects of sheet pile height h on the footing behaviour for the two cases when the footing is constructed at (b =0) and away of the slope crest distance b=B. In Fig. 10, the variations of BCI with normal-ized sheet pile height h/B for different footing

position are plotted. The figure shows that the larger is the sheet pile height, the higher is the improvement in bearing capacity. Using a sheet pile with h/B = 3, placed at slope crest brings out improvements in the bearing capacity as high as 2.62 and 1.35 times those of non-stabilized cases for the two footing positions (b=0 and b=B), respectively.

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

0 0.5 1 1.5 2 2.5 3 3.5

(h/B)

BC

I

b/B = 0.0

b/B = 1.0

Fig. 10. Variations of BCI with h/B for the two footing positions (d/B = 0.0).

This significant effect of sheet pile height

can be explained by the particles lateral defor-mations resistance mobilized by the installa-tion of sheet pile compared with that of the non-stabilized case. The longer is the sheet pile, the higher is the embedded part in the stable underlying soils, the less rotation of the sheet pile would be and hence the less lateral deformations. However, for the footing placed at (b/B = 1) the effect of the sheet pile is not

such as that when the footing placed at the crest as the bearing capacity of the non-stabilized case is relatively higher. When the footing is placed far from the crest, the passive resistance is relatively large and the contribu-tion of the sheet pile on the bearing capacity improvement seems to be lower. 4.2.2. The effect of sheet pile location

Two series of tests were conducted to investigate the best location of the sheet pile that gives the maximum bearing capacity improvement for the two footing positions. Tests using the same sheet pile (h/B = 2.0) at

five different locations relative to the slope crest (d = 0.0, 80.0, 120.0, 160 and 200.0

mm) were carried out and the test results are plotted in fig. 11. The figure clearly shows the same pattern of footing behaviour when using sheet pile as using row of piles. The most effective location of the sheet pile in terms of footing response is at the slope crest. Also, as the distance between the sheet pile and the slope crest increases the footing bearing cap-



acity decreases. The sheet pile strengthens the soil layer underlying the footing and resists the soil lateral movements leading to a subst-antial increase in bearing capacity. This effect is much pronounced with the sheet pile placed at slope crest and decreases as the sheet pile is placed far away of the crest. Also, the instal-lation of the sheet pile is significant for the footing response when the footing is placed near to the crest and that effect decreases as the distance between the footing and the slope crest increases.

4.3. Comparison of pile and sheet pile

stabilized slope

In order to find out the best method to

stabilize a sand slope in terms of the footing response, a comparison between the bearing capacity improvements of the footing for the two methods is drawn in fig. 12. It was taken into account to make the comparison for the most similar conditions; the location of piles or sheet pile and the position of the footing are the same (d/B =0 and b/B = 0). However for

piles row, the behaviour of the maximum dia-meter is considered in the comparison. The ef-fect of pile stiffness (made of steel) and the sheet pile stiffness (made of wood) was consid-ered relatively low as Lee et al. [10] stated that pile stiffness appears to have little effect on the overall pile-slope stability. The figure clearly confirms the expected trend that the sheet pile has much better effect on the foot-ing behaviour than the row of piles has. This effect is much pronounced for larger heights of sheet piles as the resistance of the sheet pile increases due to the increased embedded part of the sheet pile in stable layers of soils. 5. Theoretical analysis

Numerical study on the effect of the sheet

pile height and location relevant to the slope crest using finite element method was per-formed. The ultimate bearing capacity for both non-stabilized and stabilized prototype slopes were determined and then investigated using the normalized ultimate bearing capacity improvement BCI. The ultimate bearing capac-ity of the prototype footing for non-stabilized cases for the two footing positions b/B = 0.0

and 1.0 are 90 and 160 kN/m respectively. Figs. 13 and 14 present the failure pattern and deformed mesh for a footing placed at the crest of non stabilized slope and a footing placed at the crest of sheet pile stabilized slope with h/B = 3. Fig. 13 clearly shows the

tendency of ground movement and the footing rotation, which has significant effect on the bearing capacity, toward the slope face. Also, the observed behaviour from the experimental tests that the improvement in the bearing capacity is accompanied with an increase in the settlement ratio is confirmed by the FEA results as can be seen in fig. 14. The improve-ment obtained in the ultimate load is about

1

1.2

1.4

1.6

1.8

2

2.2

2.4

0 0.5 1 1.5 2 2.5 3

(d/B)

BC

I

b/B = 0.0

b/B = 1.0

Fig. 11.Variations of BCI with d/B for the two footing positions (h/B = 2.0).

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

0 0.5 1 1.5 2 2.5 3 3.5

(h/B) or L/B

BC

I

sheet pile, d/B=0.0, b/B=0.00

pile, d/B=0.0, b/B=0.0, D/B=0.15

Fig. 12. Comparison of the BCI with sheet pile height h/B

or pile length L/B.



Deformed Mesh

Extreme total displacement 64.26*10-3 m

(displacements scaled up 10.00 times)

Deformed Mesh

Extreme total displacement 90.71*10-3 m

(displacements scaled up 10.00 times)

60% with an increase in the S/B from 8.03 % to 11.34%. Figs. 15 and 16 show typical plots of the displacement vectors obtained from the finite element analysis for both the non-stabilized and stabilized earth slope respectively. As seen in fig. 15, the observed displacement vectors at failure for non stabilized slope are concentrated underneath the footing toward the slope face, while for the stabilized slope as shown in fig. 16, the displacement vectors spread underneath the footing along the sheet pile face for deeper depth than that in the non stabilized case. It is clear that the sheet pile wall acts as a barrier for the particle deforma-tions toward the slope face and pushes them downward for deeper depth and hence spreads the footing load deeper into the soil which in turn means a longer failure surface and greater bearing capacity.

Fig. 17 presents comparisons of the meas-ured BCI from the model tests (exper.) and the calculated BCI from the Finite Element Analy-sis FEA of prototype slope for the different

Fig. 13. Typical deformed finite element. mesh for non stabilized earth slope.

Fig. 14. Typical deformed finite element mesh for sheet pile stabilized earth slope (h/B = 3.0).

sheet pile heights and footing positions. The curves clearly demonstrate good agreement in the general trend of behaviour of both the ex-perimental and numerical analysis. The BCI increases with the increase of h/B for the two

footing positions. However, the agreement be-tween the measured and calculated values is much better when the footing is placed away of the slope crest b/B=1.0 than that when it is

placed at the crest where the difference in the BCI values appear to be high. Fig. 18 shows the variations of BCI with the location of the

sheet pile wall for both model footing and prototype slope. As illustrated, although the BCI from the FEA appears to be smaller than that for the model slope (specially for b/B=0),

the general trends of the manner in which BCI varies with the location of the sheet pile wall are in good agreement with those from the model tests. The difference between model test results and FEA can be illustrated as follow.

The physical model used in this study is small scale while the problem analyzed by the FEA is a prototype footing-slope system enc-ountered in the field. Furthermore, it is well known that due to scale effects soil may not play the same role in the laboratory models as in the prototype. However the comparison shows a close agreement in the general trend of behaviour which should be considered enc-ouraging to rely on the model scale test re-sults. So, it is recommended to carry out further investigation using full-scall tests or centrifugal model tests in order to ascertain the obtained results.

Fig. 15. Typical plots of displacement vectors for non stabilized earth slope.

6. Conclusions

Based on the experimental and theoretical

analysis, the following conclusions can be drawn:



Fig. 16. Typical plots of displacement vectors for sheet pile

stabilized earth slope (h/B = 3.0).

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

0 0.5 1 1.5 2 2.5 3 3.5

(h/B)

BC

I

b/B = 0.0 & Exper.

b/B = 0.0 & FEA

b/B = 1.0 & Exper.

b/B = 1.0 & FEA

Fig. 17. Comparisons between the experimental and the numerical results for the different sheet pile heights h/B

for the two footing positions (d/B = 0.0).

1

1.2

1.4

1.6

1.8

2

2.2

2.4

0 0.5 1 1.5 2 2.5 3

(d/B)

BC

I

b/B = 0.0 & Exper.

b/B = 0.0 & FEA

b/B = 1.0 & Exper.

b/B = 1.0 & FEA.

Fig. 18. Comparisons between the experimental and the numerical results for the different sheet pile locations

(d/B) for the two footing positions (h/B = 2.0).

1. Stabilizing earth slope using a row of piles or sheet pile has a significant effect on improv-ing the bearing capacity of a strip footing supported on granular soil adjacent to the slope crest. 2. The maximum bearing capacity is obtained when pile spacing is minimum and pile length is maximum. Increasing pile diameter further

improves the bearing capacity performance of the footing. However pile spacing has much significant effect on bearing capacity improvement than that of pile length or pile diameter. 3. The overall improvement when using sheet piles to stabilize earth slope is much better than that when using piles. The longer is the sheet pile, the higher would be the bearing capacity improvement. 4. In terms of bearing capacity improvements rather than the overall stability of the slope, the optimal location of a row of piles or a sheet pile is at the slope crest. 5. For the studied problem geometry condition, the lateral deformations of soils under the footing toward the slope are the controlling factor of the footing failure rather than the stability of the slope itself. The footing suffers not only vertical settlement but also rotation towards the slope without any potential effect on the slope stability. 6. The general trend of behaviour of the measured experimental tests is similar to that of the calculated numerical results. The difference in the bearing capacities values between the two cases may be due to scall effects. However the close agreement in the trend is encouraging to rely on the model and further investigation using full-scall tests or centrifugal model tests is recalled in order to ascertain the obtained results.

Acknowledgements

The tests were performed in Soil Mechan-ics Laboratory of Structural Engineering De-partment, University of Tanta which is ack-nowledged. The author would like to appreci-ate the valuable comments of Prof. Dr. M. A. Mahmoud. Also, the support provided by Prof. Dr. M. M. Bahloul is gratefully acknowledged.

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Received August 4, 2003

Accepted November 30, 2003

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